1. Field of the Invention
The present invention relates to an associative memory (i.e., a memory in which all words are internally searched in parallel fashion) for which wide applications are recently expected in the information processing field, and more particularly relates to a non-volatile associative memory.
2. Description of the Related Art
FIG. 8 shows an exemplary conventional associative memory which is described in "MOS/LSI Design and Application" by W. N. Carr, J. P. Mize, pp. 222-225. In this associative memory, each memory cell includes ten transistors T.sub.1 -T.sub.8, T.sub.L1 and T.sub.L2. The transistors T.sub.1 -T.sub.4, T.sub.L1 and T.sub.L2 constitute a static RAM (random access memory), so that usual read/write operations can be performed.
An associative memory action is performed in such a manner that the transistors T.sub.5 -T.sub.8 provide an exclusive-OR logic function between the voltage of the bit lines B(0) and B(1) and the node voltages Q and Q of a memory cell. For example, in the case where a search is made for data "1" while data "0" is stored in the memory cell, the transistors T.sub.6 and T.sub.8 are both conducting (positive logic). In another case where a search is made for data "0" while data "1" is stored in the memory cell, the transistors T.sub.5 and T.sub.7 are both conducting.
With the above configuration, if the contents of the memory cell do not match the contents of data to be searched, either the path containing transistors T.sub.6 and T.sub.8 or the path containing transistors T.sub.5 and T.sub.7 are conducting, which provides a conduction path from a voltage line V.sub.II to a word line W in FIG. 8. Thus, a current I.sub.M flows through a resistor R. On the contrary, if the contents of the memory cell match the contents of data to be searched, neither of the path containing transistors T.sub.6 and T.sub.8 nor the path containing transistors T.sub.5 and T.sub.7 are conducting, so that no current flows through the resistor R. Therefore, the matching of the data contents stored in the memory cell with the contents of data to be searched is performed by detecting the presence or absence of the current I.sub.M flowing through the resistor R.
As described above, the associative memory can perform the matching of the data contents stored in memory cells which are arranged along the word line W with the reference data contents in the parallel fashion, whereby the processing speed can be improved. Due to such an advantage, associative memories are applied in various fields such as pattern recognition.
However, the above-described conventional associative memory has the following drawbacks:
(1) since a single memory cell is constituted by ten transistors, the size per unit memory cell is large, whereby the associative memory is not suitable for a large capacity type; and PA1 (2) since the associative memory is not provided with means for forming a backup of the data stored in a memory cell, the contents of the memory cell will volatilize when the voltage supply is turned OFF.